1. Field of the Invention
The present invention relates to a brushless DC fan motor suitably radiating heat generated in a housing of electronic equipment, more particularly, to a drive circuit therefor.
2. Description of the Related Art
For example, in electronic equipment having a large number of electronic parts housed in a relatively narrow housing, such as OA (office automation) equipment including a personal computer, a copying machine and so on, the housing may be filled with heat generated from the above electronic parts, thereby breaking the electronic parts by heat.
Therefore, an air blower is provided in a wall surface or a ceiling surface of the housing of the electronic equipment, to which a fan motor is attached, thereby externally exhausting the heat in the housing.
While a brushless DC fan motor is often used as this type of fan motor, a conventional drive circuit for driving such a brushless DC fan motor will be shown in FIG. 4.
In the drawing, reference numeral 41 denotes a drive circuit for a brushless DC fan motor, here, which indicates a drive circuit in a brushless DC fan motor 42 of a two-phase unipolar drive type. Symbols + and − indicate the anode and the cathode of a DC source, respectively.
As shown in the drawing, the brushless DC fan motor 42 includes field coils (motor coils) L1 and L2. The field coils L1 and L2 are provided at a stator (not shown) and the flow of electric current is switched alternately by switch elements of the drive circuit 41, which are NPN transistors T3 and T4 here, to establish a rotating magnetic field. A rotor (not shown) of the fan motor 42 includes a permanent magnet, the permanent magnet being rotated following the rotating magnetic field to rotate the rotor.
The drive circuit 41 is constituted by a control circuit CC, resistors R1 to R9, PNP transistors T1 and T2, Zener diodes ZD1 to ZD4, diodes D1 and D2, and the above-mentioned transistors T3 and T4.
The control circuit CC receives a signal from a hall element 43 for detecting the location of the rotor (permanent magnet) and outputs a signal for performing an on-off control of the transistors T3 and T4. In other words, after the output signal from the control circuit CC the polarity of which has been reversed and the signal has been amplified by the transistors T1 and T2, it is input to the transistors T3 and T4, thereby performing the on-off control of the transistors T3 and T4.
Accordingly, the flow of electric current to the magnetic fields L1 and L2 is alternately switched in accordance with the location of the aforesaid rotor to produce a rotating magnetic field, and the rotor is rotated as described above, thereby externally exhausting the heat in the housing.
In such a fan motor, there is a possibility of generating a high voltage (generating a high voltage periodically) when the electricity is turned off, thereby destroying elements including the transistors T3, T4 and so on.
Accordingly, in the above-described conventional circuit, the Zener diodes ZD1 and ZD2, and ZD3 and ZD4 are connected between a collector and the base of each of the transistors T3 and T4, respectively, thereby preventing the generation of the high voltage.
However, even if the Zener diodes ZD1 and ZD2, and ZD3 and ZD4 are connected, in the conventional circuit shown in FIG. 4, a current spike flows toward the collector of the transistors T3 and T4 when the electricity of the field coils L1 and L2 is turned off, thereby causing noise.
Also, upon starting the motor, a current spike flows toward the collector of the transistors T3 and T4, causing noise.
FIG. 5 is a view of the conventional circuit for reducing a current spike to the transistors T3 and T4, as described above.
Referring to FIG. 5, the same elements as those in FIG. 4 are denoted by the same reference numerals. Here, capacitors C1 and C2 are connected in parallel between the collector and an emitter of each of the transistors T3 and T4 in FIG. 4, respectively.
With such a construction, in the conventional circuit shown in FIG. 5, a current spike which tends to flow toward the collector of each of the transistors T3 and T4 flows toward the capacitors C1 and C2, respectively, and is then absorbed. Accordingly, the noise due to the above current spike can be reduced.
However, the effect of preventing noise was not enough even in the conventional circuit shown in FIG. 5.
More specifically, the current spike which tends to flow toward the collector of each of the transistors T3 and T4 can be securely absorbed by the capacitors C1 and C2. Consequently, the noise produced by current spike flow toward the collector of each of the transistors T3 and T4 can be prevented.
However, when turning on the transistors T3 and T4 after the current spike has flowed toward the capacitors C1 and C2, a current discharge spike of each of the capacitors C1 and C2 flows toward the collector of each of the transistors T3 and T4, respectively.
In the conventional circuit shown in FIG. 5, the noise produced by such discharge current flow poses a new problem, which was brought to the fore when chip capacitors (not shown) were used as the capacitors C1 and C2.
Accordingly, one of the solving means is to not use the chip capacitors as the capacitors C1 and C2. However, the chip capacitors (not shown) are often used as the capacitors C1 and C2 in order to respond to a requirement for miniaturization of a drive circuit board. At any rate, conventionally, reduction of the noise caused by the current discharge spike from the capacitors C1 and C2 is earnestly required to realize a low noise level.